1. Field of the Invention
The present invention relates to an apparatus and method for providing an input signal having a desired pulse width and amplitude to an atomic force microscope used in nano-lithography.
2. Description of the Related Art
In general, atomic force microscopes (AFMs) are used to image the surface of a sample without causing damage to the sample. AFMs are also used in nano-lithography applications to control the arrangement of atoms or molecules on a sample surface by applying a force to damage the sample surface using a probe.
In constructing an artificial nano-sized structure by adjusting physical quantities, such as electron density or energy level, on a nanometer scale, a conventional technique such as e-beam lithography cannot be controlled on a scale less than 10 nm. AFM-based lithography is the unique technique applicable to adjust physical quantities on the scale of a few nanometers.
AFM-based nano-lithography is applied in a variety of ways, for example, to form a micro-miniature pattern using a resist material or to form an oxide pattern by applying an electric field to the surface of a hydrogen passivation silicon or polysilicon wafer with an AFM tip.
In addition, an effort to apply AFM-based nano-lithography to a next-generation data storage technique in conjunction with electrostatic force microscopy or scanning capacitance microscopy is being made (SPMxe2x80x94Scanning Probe Microscopy by Sang-il Part, Jae-won Hong, and Young-gil Nho in PAIA, p. 12).
Such AFM-based nano-lithography techniques are classified into contact type AFM-based and non-contact type AFM-based techniques.
In imaging the surface of a sample using a non-contact type AFM, the magnitude of atomic attraction is as small as on the order of 0.1-0.01 nN so that the angle at which a cantilever is bent cannot be directly measured, and thus the cantilever mechanically vibrates at a resonance frequency.
As the cantilever approaches the surface of a sample, the resonance frequency changes due to atomic attraction, and changes in amplitude and phase are measured using a lock-in amplifier (SPMxe2x80x94Scanning Probe Microscopy by San-il Part, Jae-won Hong, and Young-gil Nho in PSIA, p. 7-8).
This principle is inversely applied in nano-lithography. In particular, when a voltage is applied to the tip of the cantilever for a predetermined period of time, an oxide pattern can be formed by the electric field generated at the tip of the cantilever.
In a conventional method, a DC voltage of xe2x88x9212-12V is applied to induce an electric field. Actually, to form a 1-nm-thick oxide layer, the duration is long enough to apply a voltage for a few microseconds. Therefore, it is unnecessary to apply a continuous bias voltage such as the DC voltage. Rather, a problem of reliability occurs with the application of a continuous bias voltage, due to interaction between the tip and the surface of the sample.
To address this problem, there is a need for a technique to instantaneously apply a voltage only for lithography, but there hasn""t been enough research on this technique.
In using a contact-type AFM, a predetermined oxide pattern is formed by applying a voltage in a square wave form to the tip for a predetermined period of time, unlike when using a non-contact type AFM. A duration of a few microseconds is enough to apply a square wave to the contact type AFM. However, since the square wave has a pulse width of about 1 ms, substantially the same effects are provided as when a DC voltage is applied. Therefore, it is difficult to precisely apply nano-lithography with such a square wave applied to the conventional AFM.
To solve the above-described problems, it is a first object of the present invention to provide an apparatus for providing an input signal at a desired point of time as a square wave having a desired pulse width and amplitude to an atomic force microscope (AFM).
It is a second object of the present invention to provide a method for providing an input signal at a desired point of time as a square wave having a desired pulse width and amplitude to an AFM.
It is a third object of the present invention to provide a computer readable medium having embodied thereon a computer program for the input signal providing method.
To achieve the first object of the present invention, there is provided an apparatus for providing an input signal having a desired pulse width and amplitude to a contact type AFM for use in nano-lithography, the apparatus comprising: a pulse width adjusting unit which adjusts the width of a positive pulse of an input square wave to a predetermined pulse width; and an amplitude adjusting unit which adjusts the amplitude of the positive pulse of the square wave to a predetermined voltage.
To achieve the second object of the present invention, there is provided a method for providing an input signal having a desired pulse width and amplitude to a contact type AFM for use in nano-lithography, the method comprising: adjusting the width of a positive pulse of an input square wave to a predetermined pulse width; and adjusting the amplitude of the positive pulse of the square wave to a predetermined voltage.
The first object of the present invention is also achieved by an apparatus for providing an input signal to a non-contact type AFM for use in nano-lithography with a resonance signal, the apparatus comprising: a square pulse generating unit which generates a square pulse having a predetermined phase in synchronization with the resonance signal, and a pulse width adjusting unit which adjusts the width of a positive pulse of the square pulse to a predetermined pulse width.
The first object of the present invention is also achieved by an apparatus for providing an input signal to a non-contact type AFM for use in nano-lithography with a resonance signal, the apparatus comprising: a square pulse phase determination signal generator which generates a square pulse phase determination signal as a predetermined DC voltage according to a desired square pulse trigger point; and a first square pulse generator which generates a positive pulse of a square wave at a point of time at which the voltage of the resonance signal rises above the square pulse phase determination signal.
The second object of the present invention is also achieved by a method for providing an input signal having a desired pulse width and amplitude to a non-contact type AFM for use in nano-lithography with a resonance signal, the method comprising: generating a square pulse having a predetermined phase in synchronization with the resonance signal; and adjusting the width of a positive pulse of a square wave to a predetermined pulse width.
The third object of the present invention is achieved by computer readable media having embodied thereon computer programs for the methods for providing an input signal to a contact type and non-contact type AFM.